It is generally known to treat the surfaces of metals, such as zinc, cadmium, or aluminum with aqueous solutions that include hexavalent chromium, which contain chemicals that dissolve the surface of the metal and form insoluble films known as “chromate conversion coatings.” These hexavalent chromium-containing coatings are corrosion resistant and protect the metal from various elements which cause corrosion. In addition, it is known that chromate conversion coatings generally have good paint bonding characteristics and, therefore, provide an excellent base for paint or other finishes.
Although the aforementioned coatings enhance corrosion resistance and paint bonding properties, the coatings have a serious drawback, i.e., the toxic nature of the hexavalent chromium constituent. This is a serious problem from two viewpoints, one being the handling of the solution by operators and the other, the disposal of the used solution. Therefore, it is highly desirable to have coating solutions and coatings that are substantially free of hexavalent chromium, but at the same time capable of imparting corrosion resistance and paint bonding properties which are comparable to those imparted by conventional hexavalent chromium-containing coatings.
Of particular interest is the use of chromate conversion coatings on aircraft aluminum alloys due to excellent corrosion resistance and the ability to serve as an effective base for paint. Conventional baths used to develop these coatings contain hexavalent chromium, and it is the residual chromates in the coating that are largely responsible for the high degree of corrosion inhibition. However, these same chromates are highly toxic and their presence in waste water effluents is severely restricted. It would therefore, be desirable to provide a coating for aluminum and its alloys and for sealing of anodized aluminum utilizing relatively less toxic chemicals that could serve as an alternative to the toxic hexavalent chromate coatings. Trivalent chromium has been used in conversion coatings in addition to and instead of hexavalent chromium in an attempt to produce replacements for hexavalent chromium-containing coatings, but to date, these attempts have been only somewhat successful.
Current trivalent chromium corrosion preventive coatings are applied using chromium sulfate and potassium fluorozirconate in a working solution at a ratio of chromium:zirconium of about 0.4:1 to about 0.6:1. One drawback of this prior art solution is that the working bath is not as stable as desired. The chromium sulfate and potassium fluorozirconate-based composition is not storage stable in that, upon aging for about 1-2 weeks, a precipitate begins to form, even in unused compositions. In use, the working bath generates a significant amount of sludge that must be removed, which results in costly down-time for the processing line and disposal issues. The onset of precipitation in the bath also has a negative impact on the conversion coating formed. The conversion coatings from aged baths of the prior art have reduced corrosion resistance. Thus there is a need for an improved trivalent chromium corrosion preventive coating that overcomes these and other drawbacks of the prior art.